Work has continued on refining the technology involved in studying the toxic effects of compounds administered by inhalation. These technological capabilities are critical to conducting inhalation studies accurately and safely. Significant advances have been made in controlling the exposure atmosphere, in documenting environmental conditions, in collecting and standardizing in-life observations, and in managing the data resulting from inhalation studies. Test animals are identified with implanted microchip transponders which are read automatically by the data system. The ability to control nd accurately monitor all aspects of actual exposure has allowed studies of extremely toxic compounds, such as arsine, to be conducted safely. A two-year chronic exposure to methylene chloride is being conducted to investigate cellular and molecular processes responsible for the induction of lung and liver tumors. A study of three structurally- related compounds--styrene, alpha-methylstyrene, and divinylbenzene--will investigate leukemogenic potential and effects on pulmonary function.